feat: bundle iperf3 v3.21 binaries from davidBar-On/android-iperf3

- Add arm64-v8a and armeabi-v7a iperf3.21 binaries
- Update pubspec.yaml with assets section
- Update README with binary source information

README.md

@@ -37,29 +37,10 @@ flutter analyze
 flutter build apk --debug
 ```
 
-### Building iperf3 binaries
+### iperf3 Binaries
 
-To rebuild the bundled iperf3 binaries, use the NDK:
-
-```bash
-cd /tmp
-wget https://github.com/esnet/iperf/archive/refs/tags/3.21.tar.gz
-tar -xzf 3.21.tar.gz
-cd iperf-3.21
-
-# Configure for Android arm64-v8a
-./configure --host=aarch64-linux-android \
-  --with-openssl=/opt/android-sdk/ndk/27.0.12077973/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr \
-  --disable-sctp
-
-# Build
-make CC=aarch64-linux-android21-clang \
-     AR=aarch64-linux-android-ar \
-     AR=aarch64-linux-android-ld
-
-# Copy to project
-cp src/iperf3 /path/to/iperf3-gui/assets/iperf3/arm64-v8a/iperf3
-```
+Pre-compiled binaries are sourced from [davidBar-On/android-iperf3](https://github.com/davidBar-On/android-iperf3)
+and bundled in `assets/iperf3/` (arm64-v8a and armeabi-v7a for Android 28+).
 
 ## License